Studies were carried out this year on the genetics of irradiated vaccine induced immunity, the histopathology of the irradiated vaccine model, the explanation for incomplete killing of challenge infections in vaccinated mice, and the tissue origin of schistosome MHC antigens. In addition, in collaboration with Dr. S. James the purification and characterization of a vaccine immunogen was initiated. The resistance defect of P/N mice to vaccination with irradiated cercariae was shown to be controlled by a single gene and to be unlinked to another single gene defect controlling IgM anti-schistosomulum antibody synthesis. In a related study another IgM deficiency controlled by the Xid gene was shown not to effect vaccine induced immunity. In a separate project the failure of vaccine induced immunity to entirely eliminate infections was shown not to be due to the existence of immunologically resistant parasite sub-populations. Finally, in a histopathological study, the lung was shown to be a major site of challenge attrition in vaccinated mice. The tissue origin of schistosome MHC antigens was examined using worms recovered from radiation chimara mice. Class II (IA) antigens were shown to be hemapoeitically derived whereas Class I (K region) antigens were demonstrated to originate from a non-hemapoietic source. Mice vaccinated with frozen and thawed schistosomula plus BCG were shown to be immune to challenge infection. In collaboration with Dr. James we showed that a soluble fraction of the larvae mediates the same effect and that the vaccine immunogens are heat and pronase sensitive.